1. Field of the Invention
The present invention relates to an optical transmitter for an optical communication system, and more particularly to an optical transmitter using a polarization duobinary modulation scheme.
2. Description of the Related Art
A polarization duobinary optical signal is a binary signal having bits of ‘1’ and ‘0’. A ‘1’ bit is represented by two orthogonal polarization components each having an amplitude of 1. A ‘0’ is represented by the state at which the amplitude is zero.
The polarization duobinary optical signal can be generated by a typical transmitter for a conventional On-Off Keying (OOK) type optical communication system. It has a high resistance to a narrow signal bandwidth and a non-linear distortion of an optical fiber. The polarization duobinary optical signal has a spectrum narrower than that an OOK signal or an Amplitude Modulated Phase Shift Keying (AM-PSK) duobinary optical signal. This means that the polarization duobinary optical signal can be used for a high density Wavelength Division Multiplexing (WDM) optical communication system. Although all bits of the OOK signal or AM-PSK signal have the same polarization, ‘1’ bits of the polarization duobinary optical signal have orthogonal polarities. Therefore, the polarization duobinary optical signal is less susceptible to non-linear distortion of an optical fiber than signals based on other communication schemes.
FIG. 1 is a block diagram illustrating a structure of a conventional optical transmitter 100 for generating a polarization duobinary optical signal. The conventional optical transmitter 100 includes a differential precoder 110 for coding, dividing and outputting a binary Non-Return-to-Zero (NRZ) electric signal, a phase inverter 150 for inverting a phase of one of the electric signals divided by the precoder 110, first and second half-wave rectifier 130 and 170, first and second duobinary filter 120 and 160, first and second optical intensity modulator 140 and 180, and a Mach-Zehnder modulation unit 190.
An electric signal input to the polarization duobinary optical transmitter 100 is different from an internal signal for driving the optical transmitter 100. Therefore, the precoder 110 codes and divides the electric signal and then outputs divided signals, in order to align the electric signal with the internal signal.
The phase inverter 150 is disposed between the precoder 110 and the second duobinary filter 160. The phase inverter 150 inverts a phase of one of the divided signals and outputs the phase-inverted signal to the second duobinary filter 160.
Each of the first and second duobinary filter 120 and 160, which may be a low pass filter having a bandwidth of 0.25×transmission speed, converts the applied binary signals to ternary signals each having three logical levels of +1, 0 and −1 and then outputs the converted ternary signals. The first duobinary filter 120 is disposed between the precoder 110 and the first half-wave rectifier 130.
Each of the first and second half-wave rectifier 130 and 170 filters negative bits of the ternary signals from the first and second duobinary filter 120 and 160. The positive bits pass through the rectifier. As a result, the signals are converted back to binary signals each having bits of only +1 and 0 while passing through the first and second half-wave rectifier 130 and 170.
After passing through the first and second half-wave rectifier 130 and 170, each of the signals passes through a corresponding driver and is then input to the first optical intensity modulator 140 or the second optical intensity modulator 180.
The Mach-Zehnder modulation unit 190 includes a polarization maintaining optical fiber 191, a polarization maintaining beam splitter 192 and first and second Mach-Zehnder modulator 193 and 194. The polarization maintaining beam splitter 192 separates interferential light beams, which have been sequentially generated and input through the polarization maintaining optical fiber 191, into horizontal polarized light beams and vertical polarized light beams. The resultant signals are then output to the first Mach-Zehnder modulator 193 and the second Mach-Zehnder modulator 194, respectively.
The first Mach-Zehnder modulator 193 receives from the first optical intensity modulator 140 an electric signal having a phase which has not been inverted. This electric signal is modulated with a corresponding polarization light beam, and then output. The second Mach-Zehnder modulator 194 receives from the second optical intensity modulator 180 an electric signal having an inverted phase. This electric signal is modulated with a corresponding polarization light beam, and then output. As a result, the Mach-Zehnder modulation unit 190 generates and outputs a polarization duobinary optical signal.
FIG. 2 is a graph illustrating an example of the polarization duobinary optical signal. As noted from FIG. 2, the polarization duobinary optical signal includes X-axis polarization components and Y-axis polarization components which are orthogonal to each other.
However, in order to generate a polarization duobinary optical signal, it is necessary for the conventional transmitter to have a plurality of separate components for modulating each of the orthogonal polarization components. Therefore, the conventional polarization duobinary optical transmitter has a complicated and redundant construction which increases the volume and price of the transmitter. Further, the conventional polarization duobinary optical transmitter is problematic in that it has a symmetric construction which degrades the reliability and reproducibility of the generated polarization duobinary optical signal.